1. Field of the Invention
The present invention relates to improvements in the process of separating carbon from flyash using a triboelectric, counter current, belt type separator and more particularly to controlling the relative humidity of the flyash fed into the separator to within an optimum humidity range.
2. Description of the Related Art
Worldwide, tremendous quantities of coal are burned to generate electricity. Typically, coal is pulverized to a fine powder, pneumatically conveyed into a boiler and burned as a dispersed powder with the heat that is liberated from the burning of the powder being used to produce steam to power turbines and generate electricity. In the boiler, the carbonaceous constituents in the coal burn and release the heat. The non-combustible materials are heated to high temperatures and typically melt and pass through and out of the boiler as flyash. This flyash is typically collected prior to the flue gases going up a stack and being dispersed into the atmosphere. For example, a 1,000 megawatt power plant can burn approximately 500 tons of coal per hour. Ash levels in the range of 10% are typical of many coals burned throughout the world. It follows that flyash is produced at very high volumes throughout the industrialized world.
The economic design of any power plant is necessarily a compromise between capital costs and operating cost. The cost of equipment to grind the coal and achieve complete combustion is balanced by the value of the BTUs liberated when the coal is burned and the cost of the coal prior to being pulverized. In addition, a factor that has become important in recent years is the air pollution produced by burning coal in large utility power plants. NOx (nitrous oxide) emissions are one example of air pollution that power plants are trying to reduce. NOx is formed by oxygen and nitrogen reacting at high temperatures and is favored by high temperature. One way to reduce NOx emissions is to reduce temperatures in the boiler and to reduce excess oxygen. This is typically done through utilizing what are called Low NOx Burners. Many boiler manufacturers produce such Low NOx Burners and many utilities are in the process of installing such devices. However, an undesired side effect of reducing the temperature and excess oxygen in the burners is an increase in the unburned carbon that is in the flyash leaving the boiler.
The passage of the non-combustible minerals through the high temperature boiler and subsequent collection of the flyash, is typically followed by a quenching in boiler tube passes, which turns the relatively inert clay and shale minerals in the coal into glassy ceramic type materials. A property of these glassy inorganic particles is that they are reactive with lime to form cementacious materials. This pozzolanic property of flyash is widely exploited by the industry, i.e., flyash is incorporated into concrete where it replaces some of the cement and reacts with free lime liberated during the hydration of the cement and produces cementacious materials resulting in a stronger concrete with less free lime, rendering it sulfate resistant, stronger and cheaper. One advantage of using flyash as a pozzolan in concrete is that it turns a high volume waste into a high volume useable material. Another advantage of using flyash in concrete to displace cement is a reduction in cement production. Cement is typically produced from minerals which are sources of calcium, alumina and silica. When cement is produced, these minerals are combined in a cement kiln and heated to incipient fusion. However, for every ton of cement produced, approximately two tons of minerals are mined and approximately one ton of CO.sub.2 is emitted into the atmosphere; some of the CO.sub.2 is from the fuel and some is from the limestone used as the source of calcium. Thus another advantage of replacing cement with flyash is that it reduces CO.sub.2 emissions on a one for one basis. In particular, for each ton of flyash used, one ton less of CO.sub.2 need be emitted.
The use of flyash in concrete requires that the flyash have specific physical properties. One of these properties, defined in American Society for Testing and Materials (ASTM) C618 specifications, is a carbon content of less than 6%. However, even this specification is really an upper limit and most users want the carbon content to be as low as possible. Unfortunately, the increase in carbon in the flyash leaving the boiler due to Low NOx Burners often causes the flyash carbon level to exceed acceptable limits as defined by potential flyash users. Thus there is a tradeoff, reducing one problem, NOx in the atmosphere, exacerbates another, CO.sub.2 greenhouse emissions. Accordingly, removal of carbon from flyash, (e.g.,flyash produced from low NOx burners ) which enables the flyash to be used in concrete, benefits the utility power plant in that it avoids a waste disposal problem, benefits the concrete producer in that it uses a lower cost material than cement, and also benefits the environment in that CO.sub.2 emissions are reduced.
A number of methods have been proposed for carbon removal from flyash including low temperature combustion, froth flotation, particle size classification and electrostatic separation. Electrostatic separation encompasses a number of different technologies based upon the electrical properties of the particles being separated. One type of electrostatic separation is conductor/non-conductor separation which depends upon conductivity differences between dissimilar particles. Typically, particles are charged either by corona or through contact with a conductive surface and a rate of charge flow into or out of the particle in contact with a conductive surface determines which particles are accepted and which particles are rejected. Separators of this type are well described in the literature--see for example, Chapter 6 of the Society of Mining Engineers (SME) Mineral Processing Handbook, edited by Norman L. Weiss, copyright 1985 by American Institute of Mining, Metallurgical and Petroleum Engineers (Library of Congress catalog card number 85-072130). However, a problem common to all of these conductive/non-conductive type separators is a need for each particle to contact a conductive surface. For fine particles, the requirement to contact a conductive surface presents a number of difficulties, such as, for example, adhesion of particles to the conductive surfaces and reduction in separator capacity due to the dependence of the separator capacity on the surface area times the particle thickness.
Another type of electrostatic separation method utilizes contact charging and will hereinafter be termed triboelectric electrostatic separation. In this method, which is also described in the SME Mineral Processing Handbook, particles are charged by virtue of contact with each other. This has the advantage of not requiring contact with a conductive surface and in principal allows particles of smaller size to be separated. The SME Mineral Processing Handbook places a lower limit of 20 microns on this type of separator based on the author's practical experience. However, a triboelectric counter-current belt type separator as described by Whitlock, U.S. Pat. Nos. 4,839,032 and 4,874,507, has been successfully and consistently operated with particles much finer than 20 microns, and has been used to separate carbon from flyash (See, for example, Whitlock, (1993) "Electrostatic Separation of Unburned Carbon from Flyash "Proceedings Tenth International Ash Use Symposium, Volume 2, pp. 70-1-70-12).
The scientific and engineering literature contains extensive discussion of the importance of low ambient humidity for the observation and practice of electrostatic effects. The reason given is that films of water on solid surfaces are conductive and this surface conduction bleeds away any charge on the particles and so renders the separation ineffective. Furthermore, the literature explains that fine particles absorb moisture and can agglomerate due to that absorbed moisture. Accordingly, the combined effects of the conductive films of water and agglomerating of particles due to moisture necessitate operation of electrostatic separators in low humidity regions. For example U.S. Pat. No. 5,513,755 by Heavilon et al. discusses the importance of low humidity to avoid aggregation of the particles. In particular, Heavilon et al. discloses an electrostatic separator that charges carbon particles either by contact with a conductive belt or by induction, the charged carbon particles being released from a layer of flyash traveling on the conductive belt by means of agitation of the layer of flyash by beater bars disposed below the conductive belt. The charged carbon particles fly up into contact with an electrode and assume, by contact, an opposite charge. The oppositely-charged particle eventually moves downwardly and outwardly from the electrode into a product reject hopper or bin. Thus the electrostatic separator of Heavilon et al. is the conductor/non-conductor type described above, which depends upon the conductivity of the carbon particles to become charged and the nonconductive ash minerals to remain uncharged, and suffers from the disadvantages discussed above.
The heating of transport air used to transport flyash from a remote collection bin to, for example, an electrostatic separator and hence the heating of air used in the bulk pneumatic transport of flyash to drive off moisture is commonly practiced by the electric utility industries. Alternatively, Heavilon et al. describes the use of a heater prior to delivering the flyash to a hopper that delivers the flyash in a thin layer over the conductive belt of the electrostatic separator, the heater heats the flyash to a sufficiently high temperature, above the dew point, to drive off moisture sufficient to break the surface bond between the carbon and ash. This is a reference to a pendular state of water in an aggregation of particles described, for example, in Perry's Chemical Engineering Handbook, 6.sup.th edition Mcgraw Hill, 1984. In other words, "small amounts of liquid are held as discrete lens-shaped rings at the points of contact of the particles." The size of these lens-shaped bridges of water depends upon the surface tension (T) of water, and the amount of water present. Referring to the Kelvin equation (1) below, the surface tension (T) is a function of the pressure difference (P) or capillary suction and the radius of curvature (R) across a curved surface of the meniscus: EQU P=2T/R (1)
As discussed by W. B. Pietsch in chapter 7.2 entitled "Agglomerate Bonding and Strength," of the Handbook of Powder Science and Technology, edited by M. E. Fayed and L. Otten, 1984, Van Nostrand, Library of Congress number 83-6828, when the surface roughness of the particles exceeds the size of the pendular bond, then the liquid bridge breaks off the larger particle and the force holding the particles together decreases. Presumably, this is the moisture level necessary to "break the bond" between the carbon and the flyash.
However, Heavilon et al. are silent with respect to any measurement of moisture levels or to a specific range of moisture content level which is desirable for operation of their conductivity-based separator. In addition, the literature only discusses removal of moisture to facilitate free flow of particles and removal of moisture to avoid conductive films of moisture on non-conductive particles. It follows from the literature that low humidity would avoid both of these problems and by implication, the lower the humidity the better.